The present invention relates to keyswitches for use in computer terminals, typewriters, and the like and more particularly to an actuator assembly for use in a low profile keyswitch which includes a telescoping plunger.
Keyswitches of conventional design generally include a plunger which is slideably mounted within a housing. The plunger is designed to slide downwardly when the keytop of the keyswitch is pressed. Downward pressure on the keytop actuates an electrical switch coupled to the plunger.
For certain applications, it is desirable to have a keyswitch which is shorter than those of conventional design. Such shorter keyswitches are known as "low-profile" switches. One problem which has been encountered in designing low-profile keyswitches is that it has been difficult to maintain the desirable characteristics of prior full size keyswitches while reducing the switch height. Such desirable characteristics include full travel, such that the amount of downward movement of the keytop will be substantially the same in the low profile switch as in full size switches; a linear feel; overtravel; and hysteresis.
In a keyswitch with linear feel, the amount of force required to press the switch from its nonactuated to its fully depressed condition increases linearly as the plunger travels downwardly. In a keyboard, such as a typewriter keyboard manufactured from a plurality of keyswitches, the provision of linear feel reduces fatigue on the fingers of the keyboard operator. The use of linear feel keyswitches also increases accuracy by providing a certain degree of feedback to the keyboard operator and facilitates the efficiency of keyboard operation.
In a keyswitch with overtravel, electrical contact is made (i.e., the switch turns "on") before the keytop is fully depressed. Overtravel provides for more reliable switch operation, because keyboard operators often inadvertently fail to press the keyswitch down the whole way, particularly when typing at a high rate of speed. When overtravel is provided, the keyswitch will turn on as long as the plunger is depressed at least to the point where electrical contact is made.
The provision of hysteresis in a keyswitch results in a lag in the electrical response of the switch. For example, if the point at which a keyswitch "makes" an electrical connection on the downward stroke of the plunger is further down than the point at which the electrical connection "breaks" on the upward stroke, the switch is said to have a positive hysteresis. If the make and break points in the switch are at the exact same physical location on the downward and upward strokes (i.e., no hysteresis), it will be possible for a keyswitch operator to unintentionally produce multiple actuations if any hesitation is made at the exact moment the switch turns on. This phenomenon is known as "teasing" the switch, because what is meant as one switch actuation may be interpreted by the circuitry actuated by the switch as a plurality of actuations. Where the switch is designed such that it must pass through the make point on its return stroke before the break point is reached, such teasing, with the resultant possibility of error, will be prevented.
It would be advantageous to provide a low-profile keyswitch combining the favorable characteristics of hysteresis, overtravel, linear force, and a "full travel" of the keytop. The present invention relates to such a keyswitch.